OCA Java SE 8 Core Java APIs - Java Dates Times Format and Parse

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The date and time classes support many methods to get data out of them:

LocalDate date = LocalDate.of(2016, Month.JANUARY, 20); 
System.out.println(date.getDayOfWeek());     // MONDAY 
System.out.println(date.getMonth());          // JANUARY 
System.out.println(date.getYear());          // 2016 
System.out.println(date.getDayOfYear());     // 20 

Java has DateTimeFormatter class to format any type of date and/or time object.

What changes is the format. DateTimeFormatter is in the package java.time.format.

LocalDate date = LocalDate.of(2016, Month.JANUARY, 20); 
LocalTime time = LocalTime.of(11, 12, 34); 
LocalDateTime dateTime = LocalDateTime.of(date, time);
System.out.println(date.format(DateTimeFormatter.ISO_LOCAL_DATE)); 
System.out.println(time.format(DateTimeFormatter.ISO_LOCAL_TIME)); 
System.out.println(dateTime.format(DateTimeFormatter.ISO_LOCAL_DATE_TIME)); 

ISO is a standard for dates. The output of the previous code looks like this:

2016-01-20 
11:12:34 
2016-01-20T11:12:34 

There are some predefined formats that are more useful:

DateTimeFormatter shortDateTime = 
   DateTimeFormatter.ofLocalizedDate(FormatStyle.SHORT); 
System.out.println(shortDateTime.format(dateTime));     // 1/20/20 
System.out.println(shortDateTime.format(date));      // 1/20/20 
System.out.println(shortDateTime.format(time)); 
// UnsupportedTemporalTypeException 

The last line throws an exception because a time cannot be formatted as a date.

Example

The following statements print exactly the same thing as the previous code:

DateTimeFormatter shortDateTime = 
   DateTimeFormatter.ofLocalizedDate(FormatStyle.SHORT); 
System.out.println(dateTime.format(shortDateTime)); 
System.out.println(date.format(shortDateTime)); 
System.out.println(time.format(shortDateTime)); 

There are two predefined formats that can show up on the exam: SHORT and MEDIUM.

LocalDate date = LocalDate.of(2016, Month.JANUARY, 20); 
LocalTime time = LocalTime.of(11, 12, 34); 
LocalDateTime dateTime = LocalDateTime.of(date, time); 

DateTimeFormatter shortF = 
   DateTimeFormatter.ofLocalizedDateTime(FormatStyle.SHORT); 
DateTimeFormatter mediumF = 
   DateTimeFormatter.ofLocalizedDateTime(FormatStyle.MEDIUM); 
System.out.println(shortF.format(dateTime));      
// 1/20/20 11:12 AM 
System.out.println(mediumF.format(dateTime));     
// Jan 20, 2016 11:12:34 AM 

To use one of the predefined formats, create your own.

For example, this code spells out the month:

DateTimeFormatter f = DateTimeFormatter.ofPattern("MMMM dd, yyyy, hh:mm"); 
System.out.println(dateTime.format(f));     // January 20, 2016, 11:12 

Note

MMMM -- M represents the month. The more Ms you have, the more verbose the Java output. For example, M outputs 1, MM outputs 01, MMM outputs Jan, and MMMM outputs January.

dd -- d represents the date in the month. As with month, the more ds you have, the more verbose the Java output. dd means to include the leading zero for a single-digit month.

, to output a comma.

yyyy -- y represents the year. yy outputs a two-digit year and yyyy outputs a four-digit year.

hh -- h represents the hour. Use hh to include the leading zero if you're outputting a single-digit hour.

: -- to output a colon.

mm -- m represents the minute.

Can you figure out which of these lines will throw an exception?

1: DateTimeFormatter f = DateTimeFormatter.ofPattern("hh:mm"); 
2: f.format(dateTime); 
3: f.format(date); 
4: f.format(time); 

We have h for hour and m for minute. Remember M (uppercase) is month and m (lowercase) is minute.

We can only use this formatter with objects containing times. Therefore, line 3 will throw an exception.

Parsing Dates and Times

parse() method takes a formatter as well its parameter. If you don't specify one, it uses the default for that type.

DateTimeFormatter f = DateTimeFormatter.ofPattern("MM dd yyyy"); 
LocalDate date = LocalDate.parse("01 02 2015", f); 
LocalTime time = LocalTime.parse("11:22"); 
System.out.println(date);          // 2015-01-02 
System.out.println(time);          // 11:22 

Here we show using both a custom formatter and a default value.

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